The present invention relates to a pneumatically operated adjusting device, particularly for throttle flap adjustment with mixture compressing combustion engines, with a diaphragm held between a housing upper portion and a housing bottom portion. This diaphragm forms a vacuum-charged working chamber with the housing upper portion. With the housing bottom portion, this diaphragm forms a space open to the ambient atmosphere. The device has a guide bushing which projects from the housing bottom on the side of the diaphragm exposed to the atmosphere and is permanently connected to this housing bottom.
On motor vehicles with mixture-compressing combustion engines, which use devices actuated by intake pipe vacuum or use intake pipe vacuum as energy source (automatic drive, air conditioner), there arises the problem that when adding one of these loads, the effective intake pipe vacuum no longer meets the requirements for trouble-free operation of the engine. In order to compensate for this shortcoming, the throttle flap position is changed, increasing engine speed and restoring a desired idling condition. If now a second load of the aforementioned type is connected to the system, a renewed re-adjustment of the throttle flap is required.
An arrangement of this type is known from the German Laid-Open Document No. 26 24 420. With this continuously operating arrangement, a diaphragm unit acting on the throttle flap via a linkage the intake pipe vacuum pressure is applied as control pressure. This pressure simultaneously acts on a barometer unit which actuates a plunger; this plunger acts as movable stop for a valve located in the center of the diaphragm for venting the control pressure chamber to atmosphere. This should result in a stroke restriction on the diaphragm corresponding to the intake pipe vacuum pressure. The purpose of this arrangement is to enlarge the opening of the throttle flap when adding vacuum pressure loads.
In the installation of an automatic transmission, a certain engine speed may not be exceeded during idling since otherwise the vehicle starts to move. Therefore the maximum opening of the throttle flap must be adjusted by an adjustable mechanical stop. Thus, the stroke of the diaphragm must be limited by design.
The known arrangement could meet this requirement, but has the disadvantage that, when adding an additional load, the diaphragm assumes an intermediate position which cannot be fixed mechanically. It vibrates about this position by brief opening and closing of the valve located in the center. With this arrangement, a reproducible speed cannot be generated upon adding the second load. Also, there is the danger that the engine speed fluctuates uncontrollably.
It is the object of the present invention to provide an arrangement which ensures reproducible speeds when adding two loads consuming vacuum pressure and eliminates the uncontrolled fluctuation of the engine speed.
Another object of the present invention is to provide an arrangement of the foregoing character which is substantially simple in construction and may be economically fabricated.
A further object of the present invention is to provide an adjusting device as described which may be readily maintained in service and which has a substantially long operating life.